1. Field of the Invention
The present invention relates to a catalyst which is used for the hydrogenation of carbon dioxide or carbon monoxide in synthesizing an alcohol and/or a hydrocarbon by reacting carbon dioxide, carbon monoxide, or a carbon dioxide/carbon monoxide mixture with hydrogen gas, or which is used for steam reforming in producing hydrogen from an alcohol and water.
2. Description of the Prior Art
Investigations for the development of methanol synthesis catalysts have been made from long ago. Especially since the publication of the basic patent issued to ICI in 1968 (U.S. Pat. No. 3,790,505), catalysts constituted of copper, zinc, and aluminum oxides and produced by a coprecipitation process have been known to be highly active in methanol synthesis. Many inventions relating thereto have been made. These catalysts have been put to industrial use in plants where methanol is produced from a synthesis gas and in plants where hydrogen is produced by the steam reforming of methanol, which is the reverse reaction.
Recently, investigations are increasingly being made on the conversion of carbon dioxide to methanol with such a methanol synthesis catalyst as a measure in mitigating the problem of global warming by carbon dioxide. However, in order for carbon dioxide generated in a large quantity as in fossil-fuel combustion to be converted to methanol, it should have high convertibility sufficient to cope with extremely rapid combustion reactions. Consequently, a catalyst having even higher activity than conventional ones is earnestly desired.
For example, a reaction in the steam reforming of methanol is shown by (1). EQU CH.sub.3 OH+H.sub.2 O.fwdarw.3H.sub.2 +CO.sub.2 (1)
A reaction for methanol synthesis is shown by (2). EQU 3H.sub.2 +CO.sub.2 .fwdarw.CH.sub.3 OH+H.sub.2 O (2)
Known catalysts relating to these reactions include the following. Examples of catalysts comprising copper, zinc, and aluminum oxides and further containing at least one additive include: one containing a rare earth element or zirconium described in Japanese Patent Laid-Open No. 60-209255; one containing yttrium or lanthanide or actinide element described in Japanese Patent Laid-Open No. 60-147244; one containing chromium oxide and silver described in Japanese Patent Laid-Open No. 4-122450; one containing chromium oxide and lanthanum oxide described in Japanese Patent Laid-Open No. 5-168936; one containing gallium, vanadium, molybdenum, and tungsten described in Japanese Patent Laid-Open No. 6-312138; and one containing titanium and zirconium oxides described in Japanese Patent Laid-Open No. 8-229399. Furthermore, catalysts constituted only of three components, i.e., copper, zinc, and aluminum oxides, are described in Japanese Patent Laid-Open Nos. 50-68983, 55-106543, 56-70836, 57-130547, 57-7256, 59-222232, 59-102443, 60-190232, 60-179145, 62-53739, 3-68450, 6-170231, etc. The Examples given in these references show that the effective ranges of the proportions in the composition of copper, zinc, and aluminum oxides are from 30 to 70% by weight as Cu, from 20 to 70% by weight as Zn, and up to 15% by weight as Al, respectively.
Industrial catalysts actually used in plants for methanol synthesis and steam reforming of methanol were examined for composition and structure. As a result, these catalysts were found to have a composition in that range and have a structure comprising aluminum oxide on the order of micrometer and, coexistent therewith, copper oxide and zinc oxide on the order of tens of nanometers. Since these three components in the structure were not always effectively in contact with each other, the structure was unable to realize high activity. Therefore, for obtaining even higher catalytic activity, there has been room for improvement in catalyst structure with respect to the configuration of particles.
Among the prior art catalysts described above, those obtained by adding additives, i.e., titanium, zirconium, gallium, palladium, vanadium, molybdenum, tungsten, yttrium, lanthanide and/or actinide elements, to copper, zinc, and aluminum oxides are unsuitable for industrial use, because these additive elements are far more expensive than copper, zinc, and aluminum.
In contrast, the catalysts constituted only of copper, zinc, and aluminum oxides are satisfactory in cost. However, these catalysts in which the proportions of copper, zinc, and aluminum are in the ranges of 30 to 70% by weight, 20 to 70% by weight, and up to 15% by weight, respectively, cannot have high activity when produced by ordinary processes, e.g., the coprecipitation method. Based on data on the properties of catalysts produced by us by the coprecipitation method so as to have compositions within the above range, it is presumed that the activity of even the catalysts having the most effective composition is, at the most, about two times that of the industrial catalysts which consist of copper, zinc, and aluminum oxides and are currently used for methanol synthesis. It is therefore thought that a catalyst having a composition within the above range and having even higher performance cannot be realized unless a special production process such as that disclosed, e.g., in Japanese Patent Laid-Open No. 8-215571 is used.